<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    .box1{
      height: 0;
      width: 0;

      border-top: 50px solid black;
      border-left: 50px solid red;
      border-bottom: 50px solid yellow;

    }

  </style>
</head>
<body>
<div>
  <div class="box1"></div>


</div>


</body>
</html>